Device for measuring the purity of ultrapure water

ABSTRACT

This is a device for analyzing the quantity of organic compounds existing in a liquid, such as ultrapure water, at the outlet from a purification device including in series filter means ( 1 ), oxidation means ( 2 ) and polishing means ( 3 ), further including means for measuring the resistivity of water to determine the purity thereof, characterized in that it includes only one resistivity measuring cell ( 4 ) and in that the outlet points of said filter means ( 1 ) and said oxidation means ( 2 ) are connected to said resistivity measuring cell ( 4 ) by pipes provided with an analysis valve ( 6 ) and/or check valves ( 5 ) selectively enabling circulation of said liquid within them.

The invention relates to a device for analyzing the level of purity of aliquid obtained after purification treatment and in particular that ofso-called ultrapure water (less than 10 parts per billion (ppb))implementing that method.

The method generally employed for purifying water, or any other liquid,begins with passage through a first device including filtration andpurification means, of the type activated carbon filtering, ion exchangeresin filtering or reverse osmosis filtering. At the outlet of the firsttreatment means, the ultrapure water contains very few ions and ischaracterized by a resistivity close to, or even equal to, 18.2 MΩ·cm,but still contains organic compounds. It is then passed through a seconddevice in which those organic compounds are oxidized in order to ionizethem. During this oxidation the organic compounds are degraded and theatoms of carbon are then present in the form of carbon dioxide gas,which is dissolved in water to form bicarbonate ions HCO3⁻. Thisoxidation is obtained by passage in front of an ultraviolet lamp or byadding hydrogen peroxide.

The third purification step consists in polishing the water, i.e.passing it through an ion exchange resin that blocks the ions createdduring the preceding step and thus completes the purification of thewater. During this step the organic compounds that were not degradedduring the oxidation phase are not affected.

It remains to determine the purity of the water obtained at the end ofthis process. One method commonly employed measures its resistivity atthe outlet from the oxidation means, which is directly linked to thedissolved carbon dioxide gas content, i.e. to the number of bicarbonateions, and then determines the resistivity that it would have if theoxidation process had been continued until the end, i.e. if all itsorganic compounds had been degraded. This process, necessitating aninfinite time period, can obviously not be used and it is thereforenecessary to employ methods of estimating this limit resistivity.

There is already known, in particular from the Millipore Corporationpatent EP0581157, a device and a method for analyzing the purity ofwater that utilize the difference in the resistivity of the waterbetween the upstream and downstream sides of the oxidation means toestimate this resistivity at infinity. This method uses the device in afirst or reference mode to measure the resistivity difference betweenthe upstream side and the downstream side of the oxidation means onwater samples that have been exposed for various times to the oxidationmeans. The exposure times are typically of the order of 10, 20, 30, 40,50 and 60 seconds. By extrapolation from the curve obtained, it ispossible to determine what the resistivity of the water obtained wouldbe after an infinite exposure time, i.e. if all the carbon atoms were tobe degraded. Using an appropriate modeling program, such as the MINTEQA2program described in the publication EPA/600/3-91/021 (1991) of the USEnvironmental Protection Agency, it is possible to determine from theresistivity at infinity the total organic carbon (TOC) content in thereference water.

A second or purification or analysis mode is then used during whichultrapure water is passed at a given flowrate through oxidation means todetermine its content of organic compound impurities and thus to verifythat its purity remains nominal. The resistivity difference between theupstream and downstream sides of the oxidation means is measuredcontinuously and the total quantity of organic compounds is deducedtherefrom by means of an assumed linear relationship between theresistivity difference measured in the purification mode and theresistivity difference at infinity estimated in the reference mode.

The invention aims to propose a device for analyzing the quantity oforganic compounds existing in a liquid at the outlet from a purificationdevice that is more economical and easier to use.

To this end the device of the invention for analyzing the quantity oforganic compounds existing in a liquid, such as ultrapure water, at theoutlet from a purification device including in series filter means,oxidation means and polishing means, further includes means formeasuring the resistivity of water to determine the purity thereof, itis characterized in that said measuring means include only oneresistivity measuring is cell and in that connecting means and fluidcirculation control means are provided that are adapted to:

-   -   prohibit circulation of the fluid between the outlet point of        the filter means and the inlet point of the oxidation means        whilst authorizing circulation of the fluid between the outlet        point of the filter means and the inlet point of the resistivity        measuring cell, or    -   authorize circulation of the fluid between the outlet point of        the filter means and the inlet point of the oxidation means        whilst prohibiting circulation of the fluid between the outlet        point of the filter means and the inlet point of the resistivity        measuring cell.

The reduction to a single cell avoids the problems of uncertaintyencountered, notably when the calculation uses the value of thedifference between a number of cells, and most importantly reduces costssince, given their complexity, these cells account for a large part ofthe overall cost of the device.

According to preferred features, intended to simplify the production ofthe liquid circuit:

-   -   said resistivity measuring cell is positioned in series in the        circuit between the outlet point of the oxidation means and the        inlet point of the polishing means;    -   outlet point of the filter means is connected to said        resistivity cell by a branch circuit comprising a check valve        that is open only if the pressure at the inlet reaches a        predefined value;    -   the outlet point of the filter means is connected to the inlet        point of the oxidation means by a simple analysis valve with two        positions, open and closed;    -   the two-position valve is situated on the downstream side of the        starting point of the branch circuit;    -   the analysis valve and the measuring means are connected to a        control and calculation unit including a control module adapted        to command opening and closing of the analysis valve in a        predefined sequence and a calculation module adapted to collect        the resistivity value from said measuring cell to deduce the        purity of the water therefrom in real time.

The disclosure of the invention continues next with the description of apreferred embodiment given hereinafter by way of nonlimitingillustration and with reference to the appended drawings. In thedrawings:

FIG. 1 is a diagram of a prior art water purification device;

FIG. 2 is a diagram of a water purification device of one embodiment ofthe invention;

FIG. 3 is a curve representing the decrease of the resistivity of thewater from any value, in the reference mode, as a function of the timespent in the reactor, using a prior art approximation method; and

FIG. 4 is a curve representing the decrease of the resistivity of thewater from any value, in the reference mode, as a function of the timespent in the reactor, using an approximation method for the invention.

FIG. 1 shows a prior art water purification device comprising, inseries, filter means 1, oxidation means 2 and water polishing means 3.Two cells for measuring the resistivity of the water 4 are inserted intothe circuit respectively at the outlet of the filter means 1 and at theoutlet of the oxidation means 2.

The filter means 1 typically consist of filters based on activatedcarbon such as those marketed by the company Millipore Corporation underthe trade mark Q-GARD®. At the outlet of these filter means the waterhas a purity of approximately 10 ppb and a resistivity close to or equalto 18.2 MΩ·cm.

Here the oxidation means 2 consist of a mercury vapor UV lamp radiatingin the range from 185 to 254 nanometers. The water that flows throughthe purification device is exposed to this radiation for time periodsfrom 2 to 120 seconds.

The polishing means 3 typically consist of an ion exchange resin andproduce water with a final purity of the order of 1 to 5 ppb.

Water enters the device through the inlet point 11 of the filter means.The outlet point 12 of the filter means is connected by a pipe to theinlet point 41 of the first resistivity cell 4, the outlet of which isconnected to a three-way valve 16. This three-way valve 16 is connectedon the one hand to the inlet point 21 of the oxidation means and on theother hand to an evacuation circuit (not shown). It causes water leavingthe filter means to enter the oxidation means 2 or evacuates it from thecircuit.

The outlet point of the oxidation means 22 is connected by a pipe to theinlet point 41 of the second resistivity cell 4, the outlet point 42 ofwhich is itself connected to the inlet point 31 of the polishing means.The ultrapure water obtained is then available at the outlet point 32 ofthe polishing means.

FIG. 2 shows a water purification device of one embodiment of theinvention, comprising in series as above filter means 1, oxidation means2 and water polishing means 3. This circuit differs from the prior artcircuit in that it includes only one measuring cell 4, placed in seriesbetween the outlet point 22 of the oxidation means 2 and the inlet point31 of the polishing means 3, and in that the three-way valve 16 isreplaced by a simple analysis valve 6 with two positions, open andclosed, that is less costly and easier to use.

Water at the outlet of the filter means is divided between two pipes,one of which goes to the analysis is valve 6, as above, and a second ofwhich, forming a bypass circuit, goes directly to the resistivitymeasuring cell 4 via a check valve 5 calibrated to open above a certainvalue.

Water from the filter means 1 is directed, entirely by action on theanalysis valve 6, either to the oxidation means 2 or directly to themeasuring cell 4 via the branch circuit equipped with the check valve 5.When the analysis valve 6 is in the open position, it allows liquid topass to the oxidation means; the pressure in the branch circuit fallsand the check valve 5 remains closed. If the analysis valve 6 is closed,the pressure rises in the branch circuit and the check valve 5 opens,allowing water to pass to the resistivity measuring cell 4.

FIG. 2 also shows control means for the water purification device, whichinclude a control and calculation unit 7 and a display device 8 adaptedto provide the operator in real time with information on the puritylevel obtained. This control and calculation unit 7 controls theposition of the analysis valve 6 by means of a control module 9 andprocesses information supplied by the resistivity measuring cell 4 in acalculation module 10. The calculation module 10 executes the waterpurity calculation method and transmits the result obtained to thedisplay device 8.

As in the prior art, the method for measuring the purity of watercomprises a first measurement of the resistivity of the water at theoutlet of the filter means followed by the use of two distinct operatingmodes of the purification device, a reference mode and an analysis mode.To evaluate the resistivity of the water supplied by the filter means 1,the analysis valve 6 is closed and the pressure on the upstream side ofthe check valve increases; the valve opens when the pressure reaches thenominal opening value and the flow of liquid circulates in the branchcircuit via the check valve 5. In analysis mode or in reference mode,when the analysis valve is open, the check valve 5 prevents circulationof the liquid in the branch circuit, the pressure at its inlet remainingbelow the nominal opening pressure. The analysis valve is opencontinuously in analysis mode. In reference mode, however, it remainsclosed during predetermined time periods during which the water situatedin the oxidation means continues to be exposed to the UV radiation. Theanalysis valve is then opened to send irradiated water to the measuringcell 4. Thanks to the significantly different time periods, thereference module determines the evolution of the resistivity of thewater as a function of the time it has spent in the oxidation means.

The configuration with a check valve 5 and an analysis valve 6 meansthat a single measuring cell 4 can be used to measure the resistivity atthe outlet of the filter means 1 and at the outlet of the oxidationmeans 2. This is reflected firstly in a major saving in the cost ofproducing the device and secondly by greater ease of use, the three-wayvalves of the prior art being complicated to use under the hydraulicoperating conditions of the device.

FIG. 3 shows a number of points indicating the resistivity of the waterat the outlet of the oxidation means 2 as a function of the time that ithas spent in those means. FIG. 3 also gives a curve approximating thosepoints by an exponential function of the type ρ(t)=ρ_(∞)+(ρ₀−ρ_(∞))e^(−t/T). FIG. 4 gives the resistivity value of the same points and anapproximation curve produced by a mixed (exponential and linear)function, which can be represented as follows:

ρ(t)=ρ_(∞)+(ρ₀−ρ_(∞)) e ^(−t/T)+ρ_(slope) t+ρ _(intercept))·u_(start,length)(t)

where ρ_(slope) and ρ_(intercept) are the slope and the ordinate at theorigin of a linear function and u_(start,length)(t) is a function havingthe value 0 on a first portion of the abscissa axis corresponding to apurely exponential function and a value of 1 for the remainder of theabscissa axis where the function can be treated as the sum of anexponential function and a linear function.

The FIG. 4 curve is a better approximation of the evolution of theresistivity as a function of the UV irradiation time, especially if theUV reactor has plastic material parts that are in contact with the waterand are subjected to this radiation. The linear part of the curve takesinto account the presence in the water of organic compounds that aregenerated by the photo-ionization of these materials or by dissolvingcarbon dioxide gas from the atmosphere. This new approximation curvereduces by 50% the adjustment effected using the least-squares method.

The process leading to measurement of the purity of the water obtainedafter it passes through the purification device is described next.

The first operation is to measure the resistivity of the water at theoutlet of the filter means 1, by closing the analysis valve 6. The waterthen flows via the branch circuit and the check valve 5 directly intothe resistivity measuring cell 4, which gives the value of theresistivity of the water at the outlet of the filter means 1. This valueρ_(UPW) remains a priori constant throughout the purification operationas, it depends only on characteristics of the liquid beforepurification.

Next a series of operations in a so-called reference mode begins. Theaim of this mode is to determine the resistivity at infinity ρ_(∞REF) ofwater that will serve as a reference fluid for the remainder of themeasurements. The analysis valve 6 is open briefly, for the time toreplace water present in the oxidation means with new water coming fromthe filter means, after which this analysis valve 6 is closed. Thisvalve remains closed for a particular first time period and is thenopened so that water retained in the oxidation means passes into theresistivity measuring cell 4; the resistivity value of this water isrecorded and then the same operation is started again, varying the timespent by the water in the oxidation means. There is obtained in this waya series of measurements of resistivity as a function of time andregression techniques are used to deduce the best approximation curvepassing through these points in a diagram giving the resistivity as afunction of time. There is then obtained the resolution of theparameters of the function ρ(t)=ρ_(∞REF)+(ρ₀−ρ_(∞REF))e^(−t/T)+ρ_(slope) t+ρ_(intercept))·u_(start,length)(t), which wereunknown until now, i.e. the parameters ρ_(∞REF), ρ₀, T, ρ_(slope),ρ_(intercept) and the cut-off point of the function u_(start,length).This function with six unknown parameters makes it necessary to carryout experiments with at least six different durations. This determinesin particular the parameter ρ_(∞REF) which gives the value of theresistivity that the water would have if it had remained an infinitetime in the oxidation means, in other words if all its organiccomponents had been degraded into bicarbonate ions.

The knowledge of these two values (values ρ_(UPW) of the resistivity atthe outlet of the filter means and value ρ_(∞REF) of the resistivity atinfinity after complete oxidation) provide for starting the phase ofanalyzing the water coming from the purification device and of knowingat all times its concentration in carbon atoms, i.e. its purity level.To this end the analysis valve is left open continuously.

Water that has passed through the filter means passes with a givenflowrate through the oxidation means where it is subject to partialdegradation of its organic components and where its resistivity evolvesbecause of the dissolution of the carbon dioxide gas generated in thisway. At the outlet of the oxidation means its resistivity p is measuredby the measuring cell 4 and is a function of the residence time t duringwhich it continued to be exposed to irradiation by the oxidation means2.

Taking the single exponential curve for the resistivity evolution model,we can write ρ(t)=ρ_(∞)+(ρ_(UPW)−ρ_(∞)) e^(−t/T). The approximation,applied here, which consists in retaining only the exponential part ofthe curve for the evolution of resistivity as a function of time andthat could not have been taken into account in the reference mode withreactors including plastic material parts, is acceptable here becausethe exposure times of the water in the UV reactor remain short, whichwas not the case in the reference mode.

The value to be determined next is the value ρ_(∞) that is used toobtain the purity of the water at the outlet from the purificationdevice. This parameter is calculated by an analytical extrapolationmethod explained below.

Designating by k_(α) the ratio between the terms ρ(t) and ρ_(∞) and bye^(−α) the formula e^(−t/T), for simplicity, we obtain:

1/k _(α)=1+(ρ_(UPW)/ρ_(∞)−1) e ^(−α)

By stating that this formula applies equally to the reference mode, inthe left-hand portion of its curve (u_(start,length)(t)=0) for the sameresidence time, we obtain:

K _(REF)=ρ(t)_(REF/ρREF), and

e ^(−α)=(1−K _(REF))/K _(REF)×ρ_(∞REF)/(ρ_(UPW)−ρ_(∞REF))

It is then possible to express k_(α) as a function of ρ_(∞) andparameters that are known through using the reference mode (K_(REF),ρ_(UPW) and ρ∞_(REF))

Using the residence time t as a working parameter intended to tendtoward infinity, it is possible, using a standard iterative method, tocause k_(α) and ρ_(∞) to evolve successively until the latter parameterconverges.

There is obtained in this way the value of the resistivity that thewater in the oxidation means would have if it had remained therein foran infinite time, i.e. if oxidation of its organic compounds hadcontinued until complete.

Standard methods, for example that used by the MINTEQA2 program, thenwork back from the value of the resistivity at infinity to the totalorganic carbon (TOC) content of the water, i.e. its purity expressed inppb.

Numerous variants are possible as a function of circumstances, and inthis regard it must be pointed out that the invention is not limited tothe examples described and shown.

1. Device for analyzing the quantity of organic compound existing in aliquid, at the outlet from a purification device including in seriesfilter means, oxidation means and polishing means, further includingmeans for measuring the resistivity of water to determine the puritythereof, wherein said measuring means include only one resistivitymeasuring cell and wherein connecting means and fluid circulationcontrol means are provided that are adapted to: prohibit circulation ofthe fluid between the outlet point of the filter means and the inletpoint of the oxidation means whilst authorizing circulation of the fluidbetween the outlet point of the filter means and the inlet point of theresistivity measuring cell, or authorize circulation of the fluidbetween the outlet point of the filter means and the inlet point of theoxidation means whilst prohibiting circulation of the fluid between theoutlet point of the filter means and the inlet point of the resistivitymeasuring cell.
 2. Device according to claim 1 for analyzing thequantity of organic compounds existing in a liquid wherein saidresistivity measuring cell is positioned in series in a circuit betweenthe outlet point of the oxidation means and the inlet point of thepolishing means.
 3. Device according to claim 1 for analyzing thequantity of organic compounds existing in a liquid wherein the outletpoint of the filter means is connected to said resistivity cell by abranch circuit comprising a check valve that is open only if thepressure at the inlet reaches a predefined value.
 4. Device according toclaim 1 for analyzing the quantity of organic compounds existing in aliquid wherein the outlet point of the filter means is connected to theinlet point of the oxidation means by a simple analysis valve with twopositions, open and closed.
 5. Device according to claim 4 for analyzingthe quantity of organic compound existing in a liquid wherein saidsimple analysis valve is situated on the downstream side of the startingpoint of the branch circuit.
 6. Device according to claim 1 foranalyzing the quantity of organic compounds existing in a liquid whereinsaid simple analysis valve and said measuring means are connected to acontrol and calculation unit including a control module adapted tocommand opening and closing of said simple analysis valve in apredefined sequence and a calculation module adapted to collect theresistivity value from said measuring cell to deduce the purity of thewater therefrom in real time.